By default, each 3GL language supported by Distributed NetBeans recognizes standard source file extensions (.cob for COBOL, .for for FORTRAN, and so on).
Distributed NetBeans associates the corresponding properties, actions, and icons with these file extensions. If there are other associations you need to add or modify, you can change these default associations.
To add, remove or change file extensions, use the Tools menu, selecting Options > IDE Configuration > System > Object Types. Click on the right-column fields to add or remove file extensions.

Adding a new extension for COBOL files to the default file extensions associated with COBOL files, and object files.
Note For 3GL language files to be visible, mount a project (a directory tree under which the language files reside) in Filesystems.